linux-kernel - Re: [PATCH] uaccess: rust: add strncpy_from

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Message-ID: <aAuYyXq1FLOhKDX3@pollux>
Date: Fri, 25 Apr 2025 16:14:33 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uaccess: rust: add strncpy_from_user

On Fri, Apr 25, 2025 at 09:44:06AM +0000, Alice Ryhl wrote:
> On Thu, Apr 24, 2025 at 06:32:08PM +0200, Danilo Krummrich wrote:
> > On Thu, Apr 24, 2025 at 03:17:48PM +0000, Alice Ryhl wrote:
> > > 
> > > +/// Reads a nul-terminated string into `buf` and returns the length.
> > > +///
> > > +/// Fails with [`EFAULT`] if the read happens on a bad address. If the end of `buf` is reached,
> > > +/// then the buffer will not be nul-terminated.
> > > +#[inline]
> > > +pub fn strncpy_from_user(ptr: UserPtr, buf: &mut [u8]) -> Result<usize> {
> > 
> > Should probably be named strcpy_from_user() instead.
> 
> See my reply to Boqun.
> 
> > > +    // CAST: Slice lengths are guaranteed to be `<= isize::MAX`.
> > > +    let len = buf.len() as isize;
> > > +
> > > +    // SAFETY: `buf` is valid for writing `buf.len()` bytes.
> > > +    let res = unsafe {
> > > +        bindings::strncpy_from_user(
> > > +            buf.as_mut_ptr(),
> > > +            ptr as *const u8,
> > 
> > kernel::ffi::c_char should always match u8, but should we use the FFI type
> > regardless?
> 
> Hmm. Should we? I don't mind changing it, but I guess this could be an
> interesting discussion point.

I think we should stick to the FFI type, even if we know the types are
guaranteed to match. But I also won't object with how it is.